In mobile telephony communication systems, different types of antennas are used for transmission and reception of signals. An ordinary type of antenna is a so-called array antenna where antenna elements are arranged in adjacently arranged columns, one after the other.
Such array antennas are used in several wireless communication standards, e.g. 3GPP LTE, in order to increase system capacity, user throughput, and coverage. The performance benefits of such techniques depend to a large extent on the base station's array antenna properties. In particular, the polarization of the antenna ports and the electrical distance between the antenna ports used for transmission and reception play a vital role in what spatial multiplexing, beamforming, polarization matching and diversity gains that can be achieved in the system. A challenge in the design and choice of base station antennas is that the optimal design depends on several parameters that can change both between sites and over time on the same site. Such parameters can be different in different networks, as well as in different cells in the same network. Examples of such parameters are traffic load, spatial traffic distribution, multipath propagation characteristics, user mobility, ratio of number indoor/outdoor users, etc.
Therefore, the polarization of, and the electrical distance between, antenna ports in a base station array antenna are important design parameters for achieving optimal performance. A problem with existing such antennas is that these parameters are fixed by design. Since the optimal choice of these parameters depend on various factors, such as traffic distribution and propagation characteristics, that can change over time and be different in different cells there is a need for being able to adapt these basic multi-antenna properties to the current situation.
A typical situation today is thus that an operator deploys one or a few antenna types in the entire network and these antennas will reside there for a long time. This relatively pragmatic solution inevitably leads to that a number of compromises needs to be made in the design and choice of base station antennas, resulting in sub-optimal system performance.
There exist reconfigurable antennas which are arranged to can adapt some of their properties to changing scenarios. However, this adaptation is typically limited to radiation pattern properties such as beam width and pointing direction. These antennas are mainly used for cell shaping and load balancing between cells. This is an extremely complex control problem since there exist many inter-dependencies when changing the cell-defining beam patterns in different cells. It is therefore a desire to improving link performance without affecting the cell shape